New, exceptionally preserved fossils of an early beaked bird reveal a more complex transition from birds to dinosaurs than expected, reports a paper in this week’s Nature.
The skulls of birds are considerably different from those of their dinosaur forebears. Modern birds have an enlarged and toothless beak, bigger braincases, weaker jaw-closing muscles and more articulated skulls with mobile palates and suspended jaws. Determining how and in what order these features developed, however, has been a problem; one made difficult by the typically poor preservation of fossil bird skulls.
Bhart-Anjan Bhullar and colleagues describe four new fossils of the early bird Ichthyornis dispar, a toothy, tern-like seabird with a 60-centimetre wingspan, which lived around 100-66 million years ago in what is now North America. Ichthyornis is uniquely situated in the fossil record, being closely related to modern birds but retaining many ancestral features, including sharp, curved teeth. Although Ichthyornis was discovered in 1870, the heads of the first specimens were incomplete and badly crushed, with no new skulls uncovered until now. The new fossils are well-preserved and three-dimensional, and include one extraordinarily complete skull. The authors also surprisingly report two previously overlooked elements from the original Ichthyornis specimens.
From three-dimensional scans, the authors reconstruct the head of the bird. Like dinosaurs, Ichthyornis sports holes in its skull for large jaw-closing muscles. Overall, however, it has a largely modern-style, articulated skull - with a small, primitive beak on its jaw tips. These features would have enabled preening and object manipulation after arms became wings, and reveal that the feeding apparatus of living birds evolved earlier than previously thought.